Form: fix crash and surface clear errors for malformed XML (bug 11707)

[eduralph]

Summary

• Fixes the ValueError: not enough values to unpack (expected 2, got 1) crash in the Forms addon's FamilySection when a <section type='family'> title omits the X/Y separator (gramps-project/gramps#11707).
• Adds a pure-Python form_validator module so the loader can reject malformed XML up front, with user-facing ErrorDialogs naming the file, form, and rule that failed.
• Partially addresses #11010 (request for a user error dialog for unsupported elements) by covering its core ask: clear errors for invalid section types, missing/empty role, missing/empty type, and XML syntax errors.

What changed

Loader (Form/form.py)

• Pre-validates each definition file; syntax errors, missing required attributes, and invalid section types all surface as ErrorDialogs with the file path and a precise message.
• Partially-broken files still load their well-formed <form> entries — one bad section no longer nukes the whole file.
• Added diagnostic logging: INFO count of forms loaded per file, DEBUG per file/form trace.

Validator (Form/form_validator.py, new)

• split_family_title() — belt-and-braces helper so FamilySection never raises on a malformed title.
• validate_form_element() / validate_form_dom() — returns a list of human-readable errors; free of GTK/Gramps imports so it runs in any CI job.
• parse_and_validate() — file-level entry point that catches both parse and schema errors cleanly.

Editor (Form/editform.py)

• Uses split_family_title() instead of the raw title.split('/') that blew up in #11707.
• Added a module logger with DEBUG on EditForm open and a WARNING in FamilySection if the title lacks the separator (so future regressions are visible in logs, not just as a traceback).

Translations (Form/po/template.pot) — three new translatable dialog titles.

Tests

All green locally (38 passed, 7 subtests):

• Form/tests/test_form_validator.py — 32 pure-Python unit tests: every branch of split_family_title, every validation rule, parse_and_validate file handling, plus a sanity test that every shipped form_*.xml passes validation.
• Form/tests/test_integration_form.py — 6 integration tests that patch ErrorDialog and assert: syntax errors, invalid family titles (the exact #11707 repro), missing role, and invalid section types all raise a clear dialog; partially-broken files still load their valid forms; shipped files trigger zero dialogs.

Test plan

• python3 -m pytest Form/tests/ -v — 38 passed
• python3 -m pytest Form/tests/test_form_validator.py runs pure Python, no Gramps required
• ruff check Form/ — clean
• python3 -m py_compile Form/*.py — clean
• Manual smoke test in local Gramps 6.0 installation:
  • Shipped forms load with zero dialogs.
  • Dropping a custom.xml with an invalid <section type='Person'> shows the new ErrorDialog naming the file, form, and rule.
  • Dropping a custom.xml with the #11707 family-title repro shows a clear dialog instead of the ValueError traceback.
  • Opening a valid form from an existing source round-trips through the editor without regression.

Fixes gramps-project/gramps#11707
Refs gramps-project/gramps#11010

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